Aluminum metal-phosphate coating compositions for protecting ferrous metal alloy surfaces from oxidation and corrosion, particularly at high temperatures, are well known in the art. For example, U.S. Pat. No. 3,248,251 issued Apr. 26, 1966 to Charlotte Allen teaches a coating composition containing chromium and/or molybdenum to inhibit the reaction between an aqueous, acidic, phosphate component of the composition and a solid particulate metallic material, preferably aluminum powder.
Heretofore, phosphate-aluminum powder coating compositions for protecting ferrous alloy surfaces from oxidation and corrosion have been based on an acid-base reaction to neutralize the bonding solution and contain either hexavalent chromium or molybdenum to inhibit the oxidation of metallic aluminum. Both the hexavalent chromium and molybdenum are considered toxic chemicals and are therefore environmentally disadvantageous. In particular, hexavalent chromium is rated as a carcinogen. Molybdenum is classified as a toxic heavy metal.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a high-temperature oxidation and corrosion-resistant coating for ferrous metal alloy surfaces that does not require either chromates or molybdates to control the reaction between the bonding solution and the powdered metal, e.g., aluminum. Furthermore, it is desirable to have a bonding solution and slurry mixture of such coatings that are free of both chromium and molybdenum.